1. Field of the Invention
The present invention relates to an electromagnetic clutch for intermittently transmitting the rotating force of an external driving source, such as an engine, to a driven device, such as a compressor.
2. Description of the Related Art
An electromagnetic clutch of this type is disclosed in Unexamined Japanese Utility Model Publication No. H07-35830.
The electromagnetic clutch disclosed in this publication is applied to a refrigerant compressor. The disclosed clutch comprises a rotor rotated by an external driving source and including an electromagnetic coil, an armature plate movable between a position where the armature plate is in contact with the rotor and a position where the armature plate is spaced from the rotor, a boss coupled to the main shaft of the compressor, and leaf springs coupling the armature plate and the boss to each other.
While the electromagnetic coil is de-energized, the armature plate remains spaced from the rotor due to the elasticity of the leaf springs, and thus the rotating force of the rotor is not transmitted to the armature plate. On the other hand, when the electromagnetic coil is energized, the armature plate is attracted to the rotor against the elastic force of the leaf springs, whereby the rotor and the armature plate are coupled to each other. Because of the coupling, the rotating force of the rotor is transmitted to the armature plate and then to the boss through the leaf springs, and the rotating force of the boss is transmitted to the main shaft of the compressor.
In the case where the external driving source is an automotive engine, intermittent operation of the electromagnetic clutch permits the motive power of the engine to be intermittently transmitted to the main shaft of the compressor, with the result that the compressor is intermittently operated.
In the conventional electromagnetic clutch, however, the force of coupling between the armature plate and the rotor is derived solely from the magnetomotive force of the electromagnetic coil. Thus, to increase the torque capacity of the compressor, the electromagnetic coil and the rotor should inevitably be increased in size, hindering reducing the weight of the electromagnetic clutch as well as the consumption of electric power. Also, since the self-inductance of the electromagnetic coil increases due to its increased size, the disengagement or decoupling response of the clutch is poor, and extremely large noise is produced when the armature plate is decoupled from the rotor. Further, increase in the magnetomotive force results in a shorter synchronization time allowed at the initial stage of coupling between the armature plate and the rotor, so that the rotating force of the rotor is transmitted, as an impact force, to the armature plate, adversely affecting the internal parts of the compressor.
To solve the problem, an electromagnetic clutch has been proposed in Japanese Patent Application No. 2003-201322.
Each leaf spring used in the proposed electromagnetic clutch has an inclined portion extending at a predetermined inclination angle from the boss toward the armature plate, and the extending direction of the leaf spring intersects at an obtuse angle with the rotating direction of the armature plate.
In this electromagnetic clutch, when the rotating force of the rotor is transmitted to the armature plate, the armature plate applies a compressive force to each leaf spring toward the boss, because the extending direction of the leaf spring intersects at an obtuse angle with the rotating direction of the armature plate. Also, since the leaf spring is inclined, part of the rotating force of the rotor acts as a force of pressing the armature plate against the rotor when the compressive force is produced. This pressing force adds to the force of coupling between the armature plate and the rotor. Consequently, the actual force of coupling between the armature plate and the rotor is given by the sum of the magnetomotive force produced by the electromagnetic coil and the pressing force, and thus the magnetomotive force of the electromagnetic coil can be cut down by an amount corresponding to the pressing force.
In the second-mentioned electromagnetic clutch, a compressive force acts upon the leaf springs. It is therefore necessary that the mechanical strength of the leaf springs should be correspondingly increased, and the thickness of the leaf springs is increased for the purpose.
Although the strength of the leaf springs can be increased by increasing the thickness thereof, the magnetomotive force required to attract the armature plate to the rotor also increases, giving rise to a problem that the magnetomotive force of the electromagnetic coil cannot be significantly cut down.